The sensitization behavior in austenitic stainless steel weld metal was simulated using the phase-field method. The simulations were performed for two cases that δ-ferrite does and does not exist in austenitic phase. The calculation results revealed that the Cr-depleted zones do not form both in the vicinity of γ/M23C6 and δ/M23C6 when δ-ferrite exists. On the other hand the Cr-depleted zones form in the interface of γ/M23C6 in the austenitic stainless steel when δ-ferrite dose not exist. The suppression of the formation of Cr-depleted zone can be explained by the model that carbon atoms in the austenitic phase are consumed due to rapid growth of M23C6 in δ-ferrite.
This study examined fatigue strength of cruciform joints fabricated with laser-arc hybrid welding. Load-carrying and non-load-carrying cruciform welded joints with a plate thickness from 8mm to 25mm were tested under tensile cyclic loads and out-of-plane bending cyclic loads. Measurements of weld toe profile, residual stress and hardness around a welded portion were also conducted. The test results revealed that the fatigue strength of the hybrid welded joints is equivalent to that of arc welded joints and satisfies the fatigue design curves for arc welded joints in JSSC recommendation. Besides, the thickness effect on fatigue strength was observed in the hybrid welded joints as well as in arc welded joints.
Zinc induced cracking (ZIC) occurs under excess tensile stress even in steels suitable for hot-dip galvanizing kettles. In this study, it was examined how the excess stress affects microstructure of the base metals and heat-affected zones (HAZs), that relates to molten zinc diffusion along the grain boundary leading to ZIC. Two kinds of steels, exhibiting two-phase (ferrite and perlite) structure (Steel A) and ferrite structure (Steel B), were selected. Tensile tests were conducted with a V-shaped notch, at which a piece of zinc wire adhered, in air at 450°C and 500°C. They failed with sudden drop in flow stress at their ultimate tensile strength (i.e. excess tensile stress). The local tensile strain at the failure point was estimated based on the aspect ratio change of grains beneath the V-shaped notch before and after the tests. The Steel A specimens with the base metal and HAZ composed of ultrafine bainitic structure failed without exhibiting ZIC at similar local tensile strain at 450°C, while the specimen with HAZ composed of bainitic structure exhibited ZIC. The two-phase structure seems to be strong enough, while the ultrafine bainitic structure prevented zinc diffusion along its grain boundary maybe with carbon and/or carbon precipitates. The Steel B specimens with the base metal and HAZs exhibited ZIC and the critical strain decreased with decreasing the grain size at 450°C. The intermetallic compound layer (IMCL) formed between steels and molten zinc at 500°C was thicker than at 450°C, and thus ZIC was hardly observed in similar tests at 450°C with holding time of 20 mins before starting the test. Thus, the tests were immediately started upon reaching 500°C. Consequently, the ZIC at 500°C was divided into three groups: no ZIC in microstructure with high strength and ZIC dependent and independent on existence of IMCL.